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    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
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<body>
    <script>
        let a = +prompt('请输入一个数')
        let b = +prompt('请输入比一个更大的数')
        function num(a, b) {
            num = (a + b) / 2
            return `两个数字的平均数为${num}`
        }
        alert(num(a, b))
    </script>
</body>

</html>